The present invention relates to a lanthanum target for sputtering that has no spotty macro patterns on the surface, and to a method of producing the same.
Lanthanum (La) is one of the rare-earth elements, and is a mineral resource that is contained in the earth's crust as a mixed composite oxide. Rare-earth elements are so called because they are separated from relatively rare minerals, but they are not that rare in light of the overall earth's crust.
Lanthanum is a white metal having an atomic number of 57 and an atomic weight of 138.9, and comprises a double hexagonal close-packed structure at normal temperature.
Lanthanum has a melting point of 921° C., boiling point of 3500° C., and density of 6.15 g/cm3. Its surface is oxidized in the atmosphere, and lanthanum gradually melts in water. Lanthanum is soluble in hot water and acid. Although it is not ductile, it is slightly malleable. The resistivity is 5.70×10−6 Ωcm, and lanthanum becomes an oxide (La2O3) when burned at 445° C. or higher (refer to Dictionary of Physics and Chemistry).
With rare-earth elements, it is generally said that compounds with the oxidation number 3 are stable, and lanthanum is also trivalent. Lanthanum is a metal that is attracting attention as an electronic material such as a metal gate material or a high-dielectric (High-k) material, and research and development is recently being promoted.
Lanthanum metal is a material in which high purification is difficult to achieve since it is easily oxidized during the refining process. In addition, if lanthanum metal is left in the atmosphere, there is a problem in that the handling thereof is difficult since it will become oxidized and darkly-discolored in a short period of time.
In recent years, thinning of a gate insulator film in the next-generation MOSFET is being demanded, but with the SiO2 that has been conventionally used as the gate insulator film, the leak current will increase due to the tunnel effect, and normal operation is becoming difficult.
Thus, as a substitute for the SiO2 described above, HfO2, ZrO2, Al2O3 and La2O3 with high dielectric constant, high thermal stability, and high energy barrier against the holes and electrons in the silicon have been proposed. In particular, among the foregoing materials, La2O3 is valued highly and its electrical properties have been investigated. And its use as a gate insulator film in the next-generation MOSFET has been studied and reported (refer to Non Patent Document 1). Nevertheless, with Non Patent Document 1, the subject of research is a La2O3 film, and it does not make any reference to the properties and behavior of La element.
Moreover, Patent Document 1 relates to lanthanum primarily for use as a target material (and the production method thereof). Although it describes producing a target with lanthanum, it could not be used as a reference since there was no specific description concerning the method (conditions) of producing such a target.
It could be said that lanthanum (lanthanum oxide) is still in the research phase, but when studying the properties of such lanthanum (lanthanum oxide), if lanthanum metal itself exists as a target material for sputtering, it is possible to form a lanthanum thin film on a substrate. It will also be easy to study the behavior at the interface with the silicon substrate, and additionally study the properties of a high-dielectric gate insulator film or the like by forming a lanthanum compound, and there is also a significant advantage in that the freedom of the target as a product will increase.
Nevertheless, even if a lanthanum target for sputtering is prepared, as described above, it becomes oxidized in a short period of time (approximately 10 minutes) in the atmosphere. When an oxide film is formed on the target, the electrical conductivity will deteriorate and thereby cause defective sputtering. In addition, if the lanthanum target is left in the atmosphere for a long period of time, it reacts with the moisture in the air and becomes covered with white hydroxide powder, and it may even cause a problem of not allowing normal sputtering to be performed. Thus, measures for preventing oxidation after the target is prepared are being considered.
Nevertheless, even if the foregoing problems can be resolved, there are other problems. Specifically, at the stage of preparing a target from the molten ingot, spotty macro patterns (unevenly colored macro patterns) appear on the machined surface of the lanthanum target. FIG. 1 shows a photograph of the lanthanum target with spotty macro patterns on its surface.
In FIG. 1, the generation of spotty macro patterns (appearing like cloud) can be observed at the part slightly outside the center of target and the outlying part of target. This is a coarsened structure as shown in the Comparative Examples described later, and is an imbalanced structure in comparison to the other parts of the material.
This causes the problem of uneven deposition during sputtering and the problem of causing the generation of particles. Accordingly, it is necessary to take measures for preventing the occurrence of such spotty macro patterns, but the problem could not be resolved to date. In addition, the current situation is that there is no recognition that this kind of problem even exists in a lanthanum target.    [Non Patent Document 1] Written by Eisuke Tokumitsu and two others, “Research on Oxide Material for High-k Gate Insulator Film”, The Institute of Electrical Engineers of Japan, Research Paper of Electronic Materials, Vol. 6-13, Pages 37 to 41, Published on Sep. 21, 2001    [Patent Document 1] International Publication No. WO2009/084318